The present invention relates to a method and an apparatus for charging a secondary battery.
Recently, electrical or electronic machines have been reduced in size and weight and have been made cordless, as electronics have developed. The secondary battery, for example, a lithium (Li) secondary battery, a nickel-cadmium (Ni--Cd) battery, and a nickel-zinc (Ni--Zn) battery, or the like, is known and used as a power source of such electrical or electronic machines. Under the circumstances, it is strongly desired that the secondary battery have a long cycle life so that the secondary battery can be repeatedly charged and discharged many times.
The secondary battery generally comprises, as is well known, a positive electrode, a negative electrode, and an electrolyte. The secondary battery is conventionally provided with a separator which separates the positive electrode and the negative electrode.
The secondary battery is conventionally charged by supplying an electric direct current (DC current). Accordingly, a conventional method for charging the secondary battery comprises the steps of producing a DC current, and supplying the DC current to the secondary battery to make the DC current flow from the positive electrode to the negative electrode through the electrolyte to thereby charge the secondary battery. Thus, it is cycled that the secondary battery is charged after being discharged, so that the secondary battery can be used for a long time.
It is known in the art that dendrite crystal grows on a surface of the negative electrode when the secondary battery is charged. The growth of the dendrite crystal is accelerated as the charging-discharging cycle is repeated many times. As a result, the grown dendrite crystal often breaks through the separator and comes into contact with the positive electrode, so that the positive electrode and the negative electrode are short-circuited. Eventually, the secondary battery becomes unusable. Thus, the growth of the dendrite crystal makes the cycle life of the secondary battery short.
In case of a Li secondary battery, the short circuit between the positive electrode and the negative electrode often causes a fire. Accordingly, the growth of the dendrite crystal unfortunately renders the Li secondary battery dangerous.
When Ni--Cd and Ni--Zn batteries are rapidly charged by use of large DC current, those batteries rise in temperature by the Joule's heat due to an internal resistance of those batteries. The temperature rise inevitably deteriorates a charge acceptability on the positive electrode. As a result, the Ni--Cd and the Ni--Zn batteries are reduced in capacity.
The Ni--Cd battery suffers from another particular problem, which is referred to as a "memory effect". Namely, the Ni-Cd battery memorizes a residual discharging capacity when charging starts. After completion of charging, the Ni-Cd battery stops discharging at the memorized residual discharging capacity. Consequently, a dischargeable capacity of the Ni--Cd battery is considerably deteriorated when charged before discharge is completed up to 100% of the discharging capacity.
It is unknown why the Ni--Cd battery suffers from the "memory effect". In order to protect the Ni--Cd battery from the "memory effect", charging of the Ni--Cd battery should strictly be restricted so that the Ni--Cd battery is charged only after the discharge has completely come up to 100%. Alternatively, a charging apparatus for use in charging the Ni--Cd battery is provided with a circuit which enables the Ni--Cd battery to be charged only after it has been discharged up to 100%.